In recent years, advancement of channel codes such as turbo codes and low density parity check (hereinafter referred to as “LDPC”) codes has occurred, communication can now be performed with a low bit error rate at a low signal-to-noise ratio (SNR). Therefore, in a system for mobile communication or satellite communication, high-data rate and high-quality communication can be performed.
In a communication system that uses higher-order modulation, such as symbol mapping with the BRGC and the turbo codes or the LDPC codes, iterative decoding is applied to a receiver, thereby improving receiving performance.
Therefore, during a receiving operation, as for higher-order modulation signals, received symbol values are subject to bitwise decomposition based on probabilities, and then the results are transmitted to an iterative decoder. When a soft bit metric is generated, bit information that is required by the iterative decoder and indispensably used in the receiver is extracted.
As methods of extracting bit information from higher-order modulation signals, Log-MAP and Max-Log-MAP algorithms have been used.
It is assumed that an M-PAM signal having M received symbols is zd, and an interval between transmitted symbols is constant, and is, for example, 2d. n denotes additive white Gaussian noise, and each transmitted symbol has K=log2M bits, that is, b0, b1, . . . , bK-1.
For example, in the case of a 4-PAM signal, K=2, and in the case of an 8-PAM signal, K=4. Therefore, bits contained in a symbol may be expressed by bk (where k=0, 1, . . . , and K). Here, bk may be said to represent the position of the k-th bit. In this case, bitwise composition using received symbol values can be calculated by using the Log-MAP algorithm as represented by Equation 1.
                              Λ          ⁡                      (                          b              k                        )                          =                              ln            ⁢                                          ∑                                  A                  ∈                                      {                                                                  s                        :                                                  b                          k                                                                    =                                              +                        1                                                              }                                                              ⁢                                                          ⁢                              exp                ⁡                                  (                                      -                                                                                            (                                                      z                            -                            A                                                    )                                                2                                                                    σ                        2                                                                              )                                                              -                      ln            ⁢                                          ∑                                  B                  ∈                                      {                                                                  s                        :                                                  b                          k                                                                    =                                              -                        1                                                              }                                                              ⁢                              exp                ⁡                                  (                                      -                                                                                            (                                                      z                            -                            B                                                    )                                                2                                                                    σ                        2                                                                              )                                                                                        (                  Equation          ⁢                                          ⁢          1                )            
Here, bk denotes a k-th bit value of a received signal symbol, z denotes a received signal, A denotes a positive (+) reference signal value, B denotes a negative (−) reference signal value, and (2 denotes a signal received through an AWGN channel.
At this time, as represented by Equation 1, as the number of signal points is increased, it becomes more difficult to implement the operation structure due to an exponential operation, and thus complexity is increased. To solve this problem, the Max-Log-MAP algorithm that approximates Equation 1 is used, as represented by Equation 2.
                                                                        Λ                ⁢                                  (                                      b                    k                                    )                                            ≈                            ⁢                              LLR                ⁡                                  (                                      b                    k                                    )                                                                                                        =                            ⁢                                                1                                      σ                    2                                                  ⁡                                  [                                                                                    min                                                  B                          ∈                                                      {                                                                                          s                                :                                                                  b                                  k                                                                                            =                                                              -                                1                                                                                      }                                                                                              ⁢                                                                                                                              z                            -                            B                                                                                                    2                                                              -                                                                  min                                                  A                          ∈                                                      {                                                                                          s                                :                                                                  b                                  k                                                                                            =                                                              +                                1                                                                                      }                                                                                              ⁢                                                                                                                              z                            -                            A                                                                                                    2                                                                              ]                                                                                                        =                            ⁢                                                1                                      σ                    2                                                  ⁡                                  [                                                                                    min                                                  B                          ∈                                                      {                                                                                          s                                :                                                                  b                                  k                                                                                            =                                                              -                                1                                                                                      }                                                                                              ⁢                                              (                                                                              B                            2                                                    -                                                      2                            ⁢                            Bz                                                                          )                                                              -                                                                  min                                                  A                          ∈                                                      {                                                                                          s                                :                                                                  b                                  k                                                                                            =                                                              +                                1                                                                                      }                                                                                              ⁢                                              (                                                                              A                            2                                                    -                                                      2                            ⁢                            Az                                                                          )                                                                              ]                                                                                        (                  Equation          ⁢                                          ⁢          2                )            
Equation 2 approximates the exponential operation to the Min/Max functions. However, as the number of symbols is increased, to calculate the Max/Min values, many comparison operations are required depending on the number of cases, which causes an increase in implementation complexity.
That is, when Equations 1 and 2 are used, complexity is increased and processing speed becomes low. For this reason, an LUT (look-up table) based method is widely used. In this structure, log2M LUTs per bit are needed, and each LUT has a memory that takes the number of quantization levels into consideration. This structure is simple and has a fast response time, but it has the following drawbacks.
1. In general, a memory is used, and as the number of symbols is increased and the quantization level becomes higher, the memory amount is increased. 2. If the quantization level is lowered so as to reduce the memory amount, the performance may be deteriorated. 3. The memory is constantly activated, and accordingly current consumption is increased. 4. An area is required to implement the memory. 5. Particularly, in the case of adaptive modulation, a ROM or a ROM/RAM needs to be used, and an LUT value needs to be changed depending on a signal modulation level M to be used.
Meanwhile, in the case of QAM and PAM, a method based on approximated Max-Log-MAP has been suggested and is widely used, as represented by Equation 3.
                              Λ          ⁡                      (                          b              k                        )                          ≈                  {                                                                                          -                    z                                    ,                                                                              k                  =                  1                                                                                                                                                                                              Λ                        ⁡                                                  (                                                      b                                                          k                              -                              1                                                                                )                                                                                                            -                                          d                      k                                                        ,                                                                              k                  >                  1                                                                                        (                  Equation          ⁢                                          ⁢          3                )            
According to this method, the operation structure is simple. However, since the operation structure is a sequential operation structure in which the previous operation result is used to perform a next stage operation, there is a limitation to increase the operation speed for wideband transmission in a fast operation.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.